Mass Transfer Flashcards
The feed to fractionating column is changed from saturated vapour to saturated liquid. If the separation and reflux ratio remains unchanged, the number of ideal stages will
increase
With lapse of time, the overhead composition of light component in case of batch distillation with constant reflux
decreases
According to the film theory of mass transfer, the mass transfer co-efficient is proportional to (where,D= molecular diffusivity)
D
Dry bulb temperature of the gas is __________ the wet bulb temperature.
more than
The rough value of diffusion co-efficient of water vapor into air at 25°C may be about __________ cm2/sec.
0.25
The recovery of pencillin from the acidified fermentation broth is done by
liquid extraction
A binary mixture of benzene and cyclohexane is separated by
extractive distillation
Component A is diffusing in a medium B. The flux NA relative to a stationary point is equal to the flux due to molecular difusion, if
there is equimolar counter diffusion.
Cox chart is useful in the design of
distillation column
Absorption factor, for a fixed degree of absorption from a fixed amount of gas should be
> 1
Selectivity of the solvent used in solvent extraction should be
> 1
Fine solids can not be leached in a Pachuka tank.
Wrongggg
NSc/NPr is called the
Lewis number
The film penetration model of mass transfer was enunciated by
Toor and Marchello
The most economical range of absorption factor is
1.25 to 2
In case of __________ distillation, a solvent is added to alter the relative volatility of the mixture to be separated.
extractive
Berl saddle made of carbon cannot be used for
oxidising atmosphere
1 kg of a saturated aqueous solution of a highly soluble component A at 60°C is cooled to 25°C. The solubility limits of A are (0.6 kgA/kg water) at 60°C and (0.2 kgA/kg water) at 25°C. The amount in kg of the crystals formed is
0.2
The commonly used solvent in supercritical extraction is
carbon dioxide
The operating line lies above the equilibrium curve in case of a gas desorber.
Wrongggg
A mixture of 10% C6H6 vapour in air at 25°C and 750 mm Hg has a dew point of 20°C. Its dew point at 30°C and 700 mm Hg will be around __________ °C.
18.7
If the amount of the steam used in steam distillation is increased, the temperature of distillation
decreases
Moisture in a solid exerting an equilibrium vapour pressure equal to that of the pure liquid at the same temperature is called the __________ moisture.
unbound
When the psychometric ratio is __________ then the adiabatic saturation temperature and wet bulb temperature becomes equal.
1
Desirable value of absorption factor in an absorber is
> 1
Sticky material can be dried in a __________ dryer.
tray
__________ developed the film theory.
Higbie
Which of the following parameters remains almost constant during adiabatic saturation of unsaturated air ?
Wet Bulb Temperature
With increase in absolute humidity, the dew point of an unsaturated mixture of air and water vapor (at constant pressure and temperature)
increases
Water-ethyl alcohal solution can be concentrated (in respect of alcohal concentration) by __________ distillation.
vacuum
Extraction of pencillin employs a __________ extraction coloumn.
centrifugal
In case of gases, the binary diffusivity is proportional to (where,P= pressure)
1/P
(NSh/NRe.NSc) is termed in mass transfer operation as the
Stanton Number
For the same process conditions, the reflux ratio of an unlagged distillation column
increases in winter
Acetone is to be removed from air in an isothermal dilute absorber using pure water as solvent. The incoming air contains 5 mole% of acetone (yin= 0.05). The design equation to be used for obtaining the number of trays (N) of the absorber is, N+2 = 6 log (yin/yout).For 98% recovery of acetone, the number of trays required is/are
9
Schimdt number and Lewis number for pure air at atmospheric conditions are respectively
< 1 and ≈ 1
Diameter of randomly packed tower is normally more than 1.2 metres.
Wrongggg
Azeotropism for ethyl alcohol-water system disappears by
decreasing pressure
Heat sensitive materials with very high latent heat of vaporisation may be economically separated using
liquid extraction
An alkaline solution is used to reduce the concentration of carbon dioxide in a stream from 10% to 0.1% by absorption with irreversible chemical reaction. The overall number of transfer units based on gas phase is
4.605
JDfactor for mass transfer is a function of Reynolds number. Mass transfer by molecular diffusion from a single drop to surrounding still air is given by
Nsh = 2
Zirconia probe is used for the humidity measurement.
Wronggggg
The value ofNA/(NA+NB), for steady state equimolal counter diffusion of two gases ‘A’ and ‘B’ is
∞
With increase in the concentration of the adsorbate, the integral heat of adsorption
decreases
An air-water vapour mixture has a dry bulb temperature of 60°C and a dew point temperature of 40°C. The total pressure is 101.3 kPa and the vapour pressure of water at 40°C and 60°C are 7.30 kPa and 19.91 kPa respectively.The humidity of air sample expressed as kg of water vapour/kg of dry air is
0.152
Nitrobenzene (boiling point = 210.6°C) is steam distilled at 1 atm pressure.Nitrobenzene will distill off __________ °C.
< 100
Vanderwall adsorption is also called chemisorption.
Wronggggggg
In a moderately sized packed absorption tower, channeling (which is most severe with stacked packings) can be minimised by taking the ratio of tower diameter to packing diameter
> 8:1
Penetration model (theory) for mass transfer was enunciated by
Higbie
Surface renewal model of mass transfer was enunciated by
Danckwerts
When the __________ the relative humidity of air decreases, despite an increase in the absolute humidity.
temperature rises
For turbulent mass transfer in pipes, the Sherwood number depends on the Reynolds number as
Re0.83
Mass transfer co-efficient ‘K’ according to penetration theory varies with mass dif-fusivity as
D0.5
During constant rate drying period, vaporisation rate per unit drying surface area
does not change with time
Mass transfer co-efficient does not account for gase-liquid diffusion.
Wrongggg
For a fixed number of ideal stages in a distillation column, as the reflux ratio is increased, the difference in composition between the top and bottom product streams
increases
According to Chilton-Colburn analogy for mass transfer,NSt.NSc2/3is equal to
f/2
A pure drug is administered as a sphere and as a cube. The amount of drug is the same in the two tablets. Assuming that the shape and size do not influence the mass transfer, the ratio of rate of dissolution in water at t = 0 for the cubic to spherical tablet is
1.24
Minimum reflux ratio in a distillation column results in
maximum condenser size
In extraction, as the temperature increases, the area of hetrogeneity (area covered by binodal curve)
decreases
1 Nm3of dry air is lighter than 1Nm3of humid air.
Wrongggg
In case of an unsaturated mixture of gas and vapor, the percentage saturation is __________ its relative saturation.
lower than
At constant pressure, with increase of temperature, the dew point will
increase
For a single component absorber, the operating line is straight only when plotted in term of __________ units.
mole ratio
Molecular diffusion in solid is much faster than that in liquids.
Wronggggg
Laboratory determination of the dif-fusivity of vapor is done by __________ method.
Winkel mann’s
Unsaturated air (with dry and wet bulb temperatures being 35 and 25°C respectively) is passed through water spray chamber maintained at 35°C. The air will be
humidified
For absorbing a sparingly soluble gas in a liquid, the
liquid side coefficient should be increased.
A natural draft cooling tower is filled with wooden grids, which covers about __________ percent of the tower height.
10-15
The boiling points of pure components of a binary system __________ with increase in total pressure of the system.
increases
The change in enthalpy, when a unit quantity of gas is absorbed by relatively large quantity of adsorbent (on which a definite concentration of the adsorbed gas already exists) is termed as the
differential heat of adsorption.
Penetration theory relates the average mass transfer co-efficient (K) with diffusivity (D) as
K ∝ √D
Rate of adsorption increases as the
temperature decreases.
At 750°K and 1 atm, the approximate value of Schmidt number for air is
0.1
The velocity, concentration and temperature boundary, for the boundary layer development on a flat plate during connective mass transfer will be same, if
NSc=NPr=NLe
Which of the following is used for the concentration of rubber latex ?
Agitated film evaporator
In a single stage extraction process, 10 kg of pure solventS(containing no solute A) is mixed with 30 kg of feedFcontainingAat a mass fraction xf= 0.2. The mixture splits into an extract phaseEand a raf-finate phaseRcontainingAat xB= 0.5 and xR= 0.05 respectively. The total mass of the extract phase is (in Kg)
8.89
When the feed to a distillation column is a saturated liquid, slope of the feed line is
infinity
At the same gas flow rate, the pressure drop in a packed tower being irrigated with liquid __________ that in dry packed tower.
greater than
With increase in pressure, the relative volatility for a binary system
decreases
Absorption factor method is used to calculate the number of ideal stages, when
both operating and equilibrium lines are parallel.
What is the reflux ratio at total reflux ?
Infinity
In case of a desorber (stripper), the
operating line always lies below the equilibrium curve.
__________ column is the most suitable for achieving the best performance for mass transfer operations involving liquid with dispersed solids.
Plate
In case of an absorber, the operating
line always lies above the equilibrium curve.
The equilibrium moisture content of the solid can be reduced by increasing the absolute humidity.
Wrongguee
Air at a particular humidity is heated in a furnace. The new dew point
increases
Henry’s law states that the
partial pressure of a component over a solution is proportional to its mole fraction in the liquid.
The diffusivity (D) in a binary gas mixture is related to the temperature (T) as
D ∝ T^1.5
The slope of 9-line is determined by the
thermal condition of the feed
A distillation column with N plates is being operated under normal conditions. At some point of time, the operation is shifted to total reflux condition (i.e., no product and residue are being withdrawn and feed to the column is stopped). At the new steady state,
the top and bottom compositions correspond to the maximum enrichment available.
The dew point of an unsaturated gas-vapor mixture does not depend upon the __________ of the mixture.
temperature
Which one of the following devices is not used for both the absorption as well as the liquid-liquid extraction processes ?
Wetted wall columns
With increase in solvent rate, the number of transfer units,NtoG, for a fixed degree of absorption from a fixed amount of gas
decreases
For a binary mixture with low relative volatility, continuous rectification to get pure products will require
high reflux ratio
The temperature of water cooled in cooling tower is always __________ temperature of entering air.
more than the wet bulb
Solvent used in extractive distillation
changes the relative volatility of the original components.
The value ofL/mGratio for economical absorption operation ranges from
1.25 to 2.0
The operating line always lies below the equilibrium solubility curve in case of a single component stripper.
Wrongguee
Dryer widely used in a textile industry is __________ dryer.
conveyor
Steady state equimolal counter diffusion is encountered in
separation of a binary mixture by distillation.
The capacity of a distillation coloumn is maximum at total reflux.
Wrongguee
The value ofNA/(NA+NB) for steady state molecular diffusion of gas ‘A’ through non-diffusing gas ‘B’ is
1
When the mixture to be distilled has a very high boiling point and the product material is heat sensitive, the separation technique to be used is __________ distillation.
steam
Azeotropic distillation is employed to separate
constant boiling mixture
The drying time between fixed moisture content within diffusion controlled ‘falling rate period’ is proportional to (assuming that drying occurs from all surfaces of the solid) (where,T= thickness of the solid )
T²
Pressure drop through plate tower as compared to that through packed tower, for the same duty will be
more
Humidity chart is useful for the solution of problems concerning condensation, vaporisation and air conditioning. At a given dry bulb temperature, value of humidity obtained from the humidity chart directly, is in terms of __________ humidity.
molal
In an absorption column, the flooding velocity for random packing is __________ that for stacked/regular packing.
smaller
The fixed cost of a distillation column at minimum reflux ratio is
infinite
To get high tray efficiency
interfacial surface between liquid and gas phase should be large.
An azeotropic mixture is a __________ mixture.
constant boiling point
Large scale drying of sugar is done in a __________ dryer.
rotary
Which of the following is the most commonly used leaching solvent in vegetable oil industry ?
Hexane
Which of the following equipments is not used for gas dispersion ?
Spray towers
Which of the following remains constant during evaporative cooling process with recirculated water supply ?
Wet bulb temperature
Solvent used in extractive distillation
is of low volatility.
The diffusivity,DAB(for componentAdiffusing inB) is equal to the diffusivityDBA(for componentBdiffusing inA) for a binary mixture of
ideal gases
Mass transfer co-efficient is directly proportional toDAB^0.5, according to __________ theory.
Penetration
For a non-reacting binary mixture of ideal gases, the partial pressure distribution of both components is linear in the case of steady state equimolal counter-diffusion.
Wronguee
The surface renewal frequency in Danckwerts’ model of mass transfer is given by (KL: mass transfer co-efficient)
sqrt(KL^2 x DA)
During dehumidification of unsaturated air wet bulb temperature & partial pressure of vapor are not constant. However, during evaporative cooling process with recirculated water supply, the __________ remains constant.
wet bulb temperature
For an absorber, both equilibrium and operating line will be straight for
dilute solution and isothermal operation.
Unsaturated air (with dry bulb and wet bulb temperatures being 45°C and 25°C respectively) is passed through a water spray chamber maintained at 23°C. The air will be cooled and
humidified with wet bulb temperature decreasing.
